ISO

ISO is the numbers shown on the dial.

 The ISO is displayed here as 200 in the viewfinder.


ISO is the International Standards Organization. It represents the "standardization" of what used to be "film speed". ISO defines the sensitivity of film and the "sensitivity" of the camera sensor to light. The higher the ISO setting the more sensitive to light the camera behaves. Note the quotes around "sensitivity" as it applies to a camera sensor, I'll explain that later.

In the days of film you put a roll of film into your camera and it had only one ISO rating, it didn't change. You could over expose or underexpose the film in the camera and then "pull" or "push" the exposure during chemical processing to make up for it. But there were penalties for doing this and you had to do it for the entire roll. With film you exposed in the camera, pushed or pulled the exposure during processing, and then fine tuned the exposure in the darkroom.

Today's digital cameras have the ability to change the ISO for each and every picture, or so it would seem. However, it is better to think of the sensor as being just like film and having only one ISO. That ISO is the "Base ISO". The Base ISO is generally going to be the lowest ISO setting available that is a regular numerical value (i.e. 80 on the dial shown). Any other ISO setting requires the camera to further manipulate the electrical signal generated by the initial exposure in order to give the desired results. This is similar to "pushing" or "pulling" the ISO with film. With digital you expose in camera, push or pull the exposure during processing and recording to the card, and then fine tune the exposure in post (the "digital darkroom"). It's basically the same thing.

The higher the ISO is "pushed" the more compromises you are making and the image will begin to become noisy. Image noise is the biggest penalty for using a high ISO. (but it is not the only one, more about that in "Know your Gear"). Excessive image noise will reduce image clarity and detail.

Crows-2

Example of high ISO noise used intentionally.


ASSIGNMENT:
While maintaining proper exposure, take several pictures of the same scene while varying the ISO. Determine the highest ISO you feel is generally usable from your camera. 

This may be most easily achieved by setting your camera to "Aperture Priority" with auto ISO enabled in the menu (Nikon). Then as you change the aperture the camera will automatically adjust the ISO to keep the exposure the same.

For older cameras and point-and-shoot cameras the usable ISO limit may be 800. For high end cameras the upper limit of usable ISO may be 12,800. This is part of the reason the professional cameras cost so much more.

Know that almost every image from every camera will show some signs of noise when viewed at 100% resolution. This is known as "pixel peeping" and leads many a new photographer to question their equipment. For the average individual, and average print sizes, viewing your images at 50% resolution will give a reasonable indication of the results you will get.

Also keep in mind this result is just a "guideline" and several factors will affect the results. Noise will be more apparent in dark scenes and dark scenes will require higher ISO's.

If you eventually find, that for the types of photography you want to do, you have to push the ISO too far too often; *then* it may be time to upgrade your camera. Used or refurbished is a great way to save money on cameras and equipment. Just be sure you are buying from a reputable dealer if it's a high price item.

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